13 Jul 2023
 | 13 Jul 2023
Status: this preprint is currently under review for the journal AMT.

Closing the gap in the tropics: the added value of radio-occultation data for wind field monitoring across the equator

Julia Danzer, Magdalena Pieler, and Gottfried Kirchengast

Abstract. Globally available and highly vertical resolved wind fields are crucial for the analysis of atmospheric dynamics for the benefit of climate studies. Most observation techniques have problems to fulfill both requirements. Especially in the tropics and in the southern hemisphere more wind data availability is required. In this study we investigate the potential of radio occultation (RO) data for climate-oriented wind field monitoring in the tropics, with a specific focus on the equatorial area between ±5° latitude. In this region, the geostrophic balance breaks down, due to the Coriolis force term approaching zero. One further aim is to understand how the individual wind components of the geostrophic balance and equatorial balance approximations bridge across the equator and where each component breaks down. We analyze the equatorial balance equation within this latitude band. In a wider range over the tropics, we derive the RO wind fields also using the geostrophic approximation and we compared the RO winds with ERA5 data. From analyzing first the zonal and meridional wind component, we find that the meridional wind component is more volatile in its derivation, however the total wind speed benefits from a computation of both wind components. Investigating next the bias between the RO and ERA5 computed winds, we find that the systematic data bias is smaller than the bias resulting from the approximation itself. As a final aspect we inspected the monthly-mean RO wind data over the full example year 2009. The bias in the upper troposphere and lower stratosphere is mainly smaller than ±2 m s−1, which is in line with the wind field requirements of the World Meteorological Organization. This is encouraging for the use of RO wind fields in climate monitoring over the entire globe including the equatorial region.

Julia Danzer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-137', Anonymous Referee #1, 24 Aug 2023
    • AC1: 'Reply on RC1', Julia Danzer, 20 Oct 2023
  • RC2: 'Comment on amt-2023-137', Anonymous Referee #2, 25 Sep 2023
    • AC2: 'Reply on RC2', Julia Danzer, 20 Oct 2023

Julia Danzer et al.

Julia Danzer et al.


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Short summary
We investigate the potential of radio occultation (RO) data for climate-oriented wind field monitoring with a focus on the tropics. In this region the geostrophic balance breaks down and the equatorial balance equation may take over across the equator. Analyzing both the individual wind components and the total wind speed we found that RO wind field biases are generally smaller than ± 2 m/s, suggesting clear added value of RO for wind field monitoring.